runtime: fix GOMAXPROCS vs garbage collection bug

Fixes #1715.

R=golang-dev, rsc1, rsc
CC=golang-dev
https://golang.org/cl/4434053

src/pkg/runtime/proc.c

@@ -1196,6 +1196,12 @@ runtime·gomaxprocsfunc(int32 n)
 	if (n <= 0)
 		n = ret;
 	runtime·gomaxprocs = n;
+ 	if (runtime·gcwaiting != 0) {
+ 		if (runtime·sched.mcpumax != 1)
+ 			runtime·throw("invalid runtime·sched.mcpumax during gc");
+		schedunlock();
+		return ret;
+	}
 	runtime·sched.mcpumax = n;
 	// handle fewer procs?
 	if(runtime·sched.mcpu > runtime·sched.mcpumax) {

src/pkg/runtime/proc_test.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"runtime"
+	"testing"
+)
+
+func perpetuumMobile() {
+	go perpetuumMobile()
+}
+
+func TestStopTheWorldDeadlock(t *testing.T) {
+	runtime.GOMAXPROCS(3)
+	compl := make(chan int, 1)
+	go func() {
+		for i := 0; i != 1000; i += 1 {
+			runtime.GC()
+		}
+		compl <- 0
+	}()
+	go func() {
+		for i := 0; i != 1000; i += 1 {
+			runtime.GOMAXPROCS(3)
+		}
+	}()
+	go perpetuumMobile()
+	<-compl
+}